Electrolytic zinc processes are used to treat complex zinc-containing ores that cannot readily be treated by pyrometallurgical recovery. The usual steps in such an electrolytic process include: (a) concentrating the zinc ores; (b) roasting the zinc concentrate to eliminate sulfur and produce zinc calcine; (c) leaching the zinc calcine to provide an impure zinc sulfate solution; (d) separating the iron present usually by forming a jarosite precipitate; (e) purifying the zinc sulfate solution; and (f) subjecting the zinc sulfate solution to electrolysis to recover the zinc metal. Such a process is described in U.S. Pat. No. 4,128,617 (1978) of DeGuire et al. which is incorporated herein by reference. A simplified process flow sheet for an electrolytic zinc plant is shown in FIG. 1.
Additional details of various process modifications can be found in "The Encyclopedia of Chemical Technology", Kirk-Othmer, Vol. 24, pp. 812-824, 3rd Ed, incorporated herein by reference.
As indicated above, a common method of removing the iron present in the leachate is through the formation of a "jarosite" precipitate. Jarosite, MFe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 where M is a monovalent ion, usually an alkali metal (general sodium or potassium) or ammonium, is commonly formed by adding a source of ammonium or sodium ions to the leach solution and maintaining the solution at an appropriate pH by the addition of base. This process is also shown in FIG. 1. Various modifications to the so-called "jarosite process" are discussed in the article entitled "The Jarosite Process--Past, Present and Future", V. Arregui et al., Lead-Zinc-Tin, TMS-AIME World Symposium on Metallurgy and Environmental Control, 1980, J. M. Cigon, T. S. Mackey and T. J. O'Keefe, Ed., pp. 97-123, incorporated herein by reference.
There are a number of problems associated with the formation of jarosite waste material. The jarosite can contain valuable metals such as silver, zinc, copper, lead, indium, etc. which require numerous expensive process steps to recover. The jarosite can also contain toxic species which can be leached into the environment by rain and groundwater. Therefore to avoid environmental contamination it is usually necessary to store the jarosite wastes in sealed lagoons which are expensive to build.
A number of methods for treating such wastes have been disclosed. Steintveit et al. in Norwegian Patent 142,406 (1980) disclose a process for leaching iron-containing waste with chloride-containing, acidic solution at a temperature between 50.degree. C. and the boiling point of the solution. An alkali metal chloride or an alkaline earth metal chloride is used as the source of the chloride with calcium chloride being disclosed as the preferred material. The pH of the solution is adjusted during the leaching step so that the iron remains as a precipitate while the valuable metals are leached into the hot solution. The pH is adjusted to between about 2 and 4 preferably with calcium hydroxide. It is disclosed that these conditions allow extraction of up to 95 percent of the lead and silver content of the waste. However, Applicants have found that unpredictably with some jarosite wastes this process provides recoveries of less than 20 percent of the silver present.
U.S. Pat. No. 4,054,638 of Dreulle et al. (1977) is directed to a process for recovering metals from sulfated residues from electrolytic zinc plants. The residue is digested preferably at a temperature between 95.degree. and 115.degree. C. with hydrochloric acid in the presence of calcium chloride. This leaching process dissolves the metals present, including the iron, by forming the corresponding metal chlorides. Consequently, the process requires that the iron chloride be removed by extraction by an organic solvent. This process has a disadvantage of solubilizing the iron and requiring a separate separation step. There is no suggestion or disclosure of using superatmospheric pressure for this leach.
U.S. Pat. No. 4,070,437 of Van Ceulen (1978), discloses a process for recovery of metals from jarosite sludges. The process involves leaching the jarosite with an acidic calcium chloride solution, preferably formed by mixing hydrochloric acid and calcium hydroxide or calcium carbonate. The leaching is preferably carried out close to the boiling point of the leaching medium. Insoluble calcium sulfate is formed and is separated by filtration. This process has the disadvantage of solubilizing essentially all of the iron in the jarosite.
Another waste which contains metal values is zinc ferrite-containing materials. Modern electrolytic zinc processes commonly use a two-step leaching process as depicted in FIG. 1. The second leaching step involves a hot acid leach to dissolve zinc ferrite present. However, prior to the development of the two-step leach, a single neutral leach was used which caused much of the zinc ferrite and associated metal values to be discarded as wastes. Therefore, there are existing waste lagoons which contain substantial quantities of zinc ferrite and other metal values. The term "ferrite" is used herein to refer to a combined metal oxide-ferric oxide material, e.g. zinc ferrite (ZnO.Fe.sub.2 O.sub.3).
A number of processes have been developed for the purpose of recovering this zinc. One such process is disclosed by Rastas et al. in U.S. Pat. No. 3,959,437 (1976). Rastas et al. disclose a process in which the ferrite of a non-ferrous metal, as well as the oxide of the non-ferrous metal, is subjected to a neutral leach which dissolves most of the oxide but leaves the ferrite substantially unaffected. The non-ferrous values in the solution are recovered and the undissolved ferrite material is further treated in a "conversion" stage with sulfuric acid-bearing solution at atmospheric pressure and at a temperature of about 80.degree. C. to about 105.degree. C. in the presence of alkali or ammonium ions. Under these conditions, the non-ferrous metals dissolve as sulfates, while iron is simultaneously precipitated as an insoluble complex sulfate, i.e., jarosite. U.S. Pat. No. 4,355,005 of Rastas et al. (1982), U.S. Pat. No. 4,366,127 of Rastas et al. (1982), as well as U.S. Pat. No. 4,383,979 of Rastas et al. (1983) each disclose modifications to the process disclosed in the '437 patent.
Steintveit in U.S. Pat. No. 3,684,490 (1972) discloses a method for treating jarosite residue in which the residue is subjected to leaching with sulfuric acid at a temperature of 50.degree. to 95.degree. C. and an acid concentration of 10 to 70 grams per liter (hereinafter g/l). These leaching conditions are intended to decompose any zinc ferrites present and provide for a greater recovery of the zinc.
U.S. Pat. No. 3,691,038 of Von Roepenack et al. (1972) discloses a method for recovering zinc from oxides containing zinc and iron. The oxide is leached with sulfuric acid at a temperature of 95.degree. to 100.degree. C. with an excess of sulfuric acid to solubilize the zinc and iron. Alkali metal or ammonium ions are added to the liquid phase along with a zinc-containing oxidic material at a temperature of 95.degree. to 100.degree. C. to precipitate jarosite.
U.S. Pat. No. 4,192,852 of Pammenter et al. (1980) discloses a process for treating zinc plant residues containing zinc ferrite and precipitating the iron as a jarosite. The sulfate solution containing ferric iron, free acid and non-ferrous metals is cooled, partially neutralized and then heated to a temperature not exceeding the boiling point at atmospheric pressure in the presence of sodium, potassium or ammonium ions. U.S. Pat. No. 4,305,914 of Pammenter et al. (1981) discloses a process similar to that in the '852 patent.
U.S. Pat. No. 4,128,617, of DeGuire et al. (1978), describes a three-step process for the treatment of zinc calcine containing zinc oxide, zinc sulfates, and zinc ferrites. The first step involves the neutral leaching of the zinc calcine with an effective amount of aqueous sulfuric acid containing solution. The leach residue is subjected to hot acid leaching with sulfuric acid followed by jarosite precipitation by alkali with the subsequent recycling of the jarosite-containing pulp. The preferred temperature range for the hot acid leaching is from about 80.degree. C. to the boiling point and preferably the temperature is greater than 90.degree. C. There is no suggestion of the use of pressure.
The processes described hereinabove have one or more of the disadvantages of (1) having low rates of extraction of metal values, (2) providing low levels of recovery of certain metal values, (3) having poor filter-ability of the iron-containing residue, and/or (4) solubilizing large amounts of iron.
Several processes disclosed in the art have used elevated temperature and pressure leaching steps in treating zinc plant residues.
U.S. Pat. No. 3,143,486 of Pickering et al. (1964) discloses a process for the extraction of zinc from zinc ferrite containing residue. The process involves subjecting the residue to a first-stage leaching treatment under non-oxidizing conditions in a closed vessel in the presence of excess sulfuric acid at a temperature between 140.degree. C. and 260.degree. C. Zinc is dissolved as well as ferrous sulfate which is stable at the temperatures and acidities used. Ferric iron is precipitated as a basic sulfate. The leachate is then subjected to a second-stage leaching treatment at 140.degree. C. to 260.degree. C. under oxidizing conditions to oxidize the ferrous sulfate to ferric and precipitate the ferric material. Similarly, U.S. Pat. No. 3,493,365 of Pickering et al. (1970) discloses a two-step high temperature method of treating zinc plant residue containing zinc ferrite. This process differs from that of the '486 patent in that in the second step a source of a cation selected from the group consisting of sodium, potassium and ammonium is added in order to precipitate the iron from the liquor as a jarosite material.
A process for treating sulfide ores which involves a two-step leach is disclosed by U.S. Pat. No. 4,266,972 of Redodno-Abad et al., (1981). The first leach uses sulfuric acid under an oxygen atmosphere at 150.degree. to 250.degree. C. Zinc and copper are solubilized with lead, the noble metals, and iron oxide remaining as a residue. After a solid liquid separation, the filtrate is adjusted to a pH of 1.5 to 2. Sodium chloride, calcium chloride and ferric chloride are added to precipitate calcium sulfate. The leach is conducted at a temperature between 60.degree. C. and 90.degree. C. with the silver, lead, and gold being solubilized as the chlorides. The iron oxide remains as a residue. After a solid/liquid separation, the silver, lead, and gold are recovered by cementation with zinc, with the liquid being subjected to an extraction to recover the zinc.
None of these processes which use high temperature and pressurized leaches discloses or suggests that jarosite-containing wastes can be advantageously treated in such a manner. In fact, most disclose the use of an oxidizing atmosphere to form ferric iron which will precipitate. These patents also disclose that potassium ions can be added to a zinc ferrite leach solution in order to precipitate potassium jarosite. As discussed in detail hereinbelow, it has been found that the recovery of metal values can be unpredictably affected by the presence of potassium ions.
Accordingly, there is a need for a process to treat jarosite and ferrite containing wastes from zinc recovery processes in order to recover metal values which are contained in the waste materials and render the residue suitable for disposal as a nonhazardous waste. There is also the need for a process which will not be subject to the unpredictable effect of potassium ions.